Is caffeine a threat to hydration or rehydration?

We all know that caffeine is a diuretic. A cup of coffee makes us dash to the restroom and for every cup we drink, we need to add another glass of water to our daily eight. That can’t be good for athletes who are already losing buckets of sweat during exercise, right? Or for the plane flight home after the race? After all, the airline magazine on your flight home carries a warning along the lines of “drink tea and coffee in moderation since these are diuretics and will increase your dehydration,” so it must be serious. Drinking caffeinated drinks before and during exercise on a hot day is particularly risky because your performance will go down the toilet. The threat to hydration or rehydration probably rates as one of the best known facts about caffeine.

Except that it’s not true. Several experts have recently tried to debunk this old wives’ tale about caffeine and dehydration. We like the work of Professor Lawrence Armstrong because it takes into account issues related to exercise. Here are some of his observations from 2002 and 2005.

A diuretic is a substance that increases the production of urine. By definition, water or any drink consumed in large volumes is a diuretic. The effect of any fluid on body hydration is judged by the balance sheet of how much the body retains of any volume that is consumed.

Caffeine is a weak diuretic. It may influence the volume of urine production and loss by acting on some of the hormones involved in urine production, and it may also influence the loss of electrolytes in urine. However, tolerance to the diuretic effects of caffeine is acquired in as little as 4 or 5 days of regular caffeine intake. Studies in the 1920s showed that for someone who hadn’t consumed caffeine in more than 60 days, a dose as little as 0.5 g/kg (e.g., 35 mg for a 70 kg person) caused a noticeable increase in urine losses. However, regular caffeine intake created a tolerance to the diuretic effect such that a dose of 1.12 g/kg was needed before an effect on urine losses was detectable.

A variety of studies have compared urine losses when subjects consumed caffeine with or as part of a beverage compared with water or a noncaffeinated control or placebo condition. Most were undertaken in sedentary conditions, and a couple were undertaken after exercise or during exercise. Caffeine doses ranged from low (~100 mg) to high (~700 mg) and urine losses were tracked for several hours (up to 24 hours). In some cases, the differences in urine losses between caffeine and no caffeine were judged to be similar; in others, the urine losses in the caffeine trial were significantly greater according to a statistical interpretation. Small to moderate intakes of caffeine (

When healthy people were followed over 11 days in which their caffeine intake was first stabilized for 6 days and then manipulated for 5 days at zero, low (3 mg/kg), or moderate (6 mg/kg) levels, there was no significant effect on urine production or characteristics.

Armstrong concluded that the consumption of caffeine doesn’t pose a threat to hydration levels via its alleged diuretic actions, particularly in small to moderate doses, and that any effects on urine losses are small and unimportant. A systematic review by Professor Ron Maughan, who we introduced in the preface of this book, came to a similar conclusion (Maughan and Griffin 2003). The summary to this publication stated that the available literature suggests that acute ingestion of caffeine in large doses (at least 250-300 mg) . . . results in a short-term stimulation of urine output in individuals who have been deprived of caffeine for a period of days or weeks. A profound tolerance to the diuretic and other effects of caffeine develops, however, and the actions are much diminished in individuals who regularly consume tea or coffee. Doses of caffeine equivalent to the amount normally found in standard servings of tea, coffee and carbonated soft drinks appear to have no diuretic action. . . . The most ecologically valid of the published studies offers no support for the suggestion that consumption of caffeine-containing beverages as part of a normal lifestyle leads to fluid loss in excess of the volume ingested or is associated with poor hydration status. Therefore, there would appear to be no clear basis for refraining from caffeine-containing drinks in situations where fluid balance might be compromised.

Before leaving the issue of caffeine and hydration, we should examine two specific variations on this theme. Urine losses contribute to fluid balance, of course, but the big picture is far more complicated than that. The minus side of fluid balance also includes sweat losses, and it has been proposed that caffeine intake might increase sweat rates either by directly affecting the sweat glands or by raising metabolic rate and body temperature. This might be of greatest importance to consider when an athlete is exercising in hot weather, and caffeine use might have an effect on heat regulation as well as hydration. Several studies have examined this issue, and in addition to measuring the individual components of fluid balance, they have looked at the bottom line of total fluid balance, body temperature regulation, and performance. The findings have been consistent: Caffeine intake before and during exercise has zero to minor effect on sweat losses, body temperature, and urine losses during exercise. However, there is no detriment to total hydration, temperature control, or performance compared with trials without caffeine. Therefore, decisions to use moderate doses of caffeine before and during exercise in hot conditions need not be overly concerned about heat and hydration.

The final issue that we need to address regarding caffeine and hydration is the plus side of the fluid balance sheet. A key consideration in real life is that a drink needs to be consumed before it contributes to body fluid levels. As much as thirst is important, people also drink according to habit, social behavior, fluid availability and personal preferences. Caffeine-containing drinks score highly in each of these areas. Beverages such as tea, coffee, cola drinks, and in some populations, energy drinks, contribute a significant volume of our daily voluntary intake of fluid. In some cases, there may be good reasons to suggest we should reduce our total or specific intake of these drinks. We now know that fluid balance isn’t likely to be one of these reasons, although we know of people who swear that they have to get up in the night to go to the bathroom if they drink caffeine-containing drinks before bed. But, we also need to recognize the value of the fluid that they do contribute to our daily balance. If people were to suddenly give up their normal intake of caffeinated beverages, the net effect might be a reduction in fluid intake until they found other drinks that were similarly enjoyable, ubiquitous, and easy to ingrain into their habits and social routines. The Perfect Study sidebar illustrates a case in point.